Cloyster, The Bivalve Pokémon. It fights by keeping its shell tightly shut for protection and by shooting spikes to repel foes. Once it slams its shell shut, it is impossible to open, even by those with superior strength. Cloyster that live in seas with harsh tidal currents grow large, sharp spikes on their shells.

Overview

Cloyster is a cool guy. Definitely one of my favourites (although some people think Shellder is cooler...). Back in 3rd gen is when it was really useful, being one of the few Pokémon to get Spikes and Rabid Spin, as well as being a strong physical wall. And then... Stealth Rock came into existence. Not only that, but everything got stronger moves such as Tyranitar upgrading to Stone Edge from Rock Slide, Fighting types upgrading from Brick Break to Close Combat, and pretty much everything being able to run mixed sets, which really destroyed Cloyster's usefulness as a spinner and a wall. So, this left Cloyster with being offensive as the only option, which with Skill Link it did quite well. However, when 5th gen was released, Cloyster got Shell Smash. Yeah, this one move skyrockets Cloyster's usability, and gives it the power to OHKO like... half of the OU tier. This overview is already huge so I won't go on about it but... +2 Attack, +2 Special Attack and +2 Speed at the cost of -1 Defence, which is stupidly huge so isn't a massive issue, and -2 Special Defence, which is stupidly low so isn't an issue either.

The most common Cloyster set, and for good reason. It's amazing. Shell Smash brings your offensive stats to crazy levels, but as the cost of bulk... bulk which Cloyster doesn't have and hasn't used since 3rd gen. Icicle Spear has 187 base power when you factor in STAB, which is ridiculous. It's stronger than STAB Blizzard. Rock Blast gets pretty good coverage with Icicle Spear, and is a strong move regardless. The last move is a tossup. Razor Shell is Cloyster's only good physical Water attack, and it is a good one. However, running a mixed set if often the best idea. Surf is more reliable than Hydro Pump, but it's nowhere near as strong. Hydro Pump allows you to take out common physical walls like Forretress before it can Gyro Ball you. Ice Shard is there to revenge kill stuff, and for hitting Breloom before it can Mach Punch you.Life Orb gives you some serious power. Focus Sash allows Cloyster to work as a lead and start decimating from the very start. King's Rock may seem dumb, but it effectively gives you a 47% chance to flinch when using Icicle Spear and Rock Blast. Yeah, that's insane. White Herb means you don't lose your defence, but eh... I'd rather decimate stuff harder.

Outdated as hell, but it can still be decent... I guess. Spikes or Toxic Spikes gives you an entry hazard and makes your opponent think twice about switching. Rapid Spin clears your field of entry hazards, which is always nice. Ice Shard helps revenge kill Dragons, whereas Ice Beam is a stronger attack. Razor Shell is your physical Water move, but Surf is stronger.

Same set, different moves. Shell Smash is amazing and makes Cloyster a God. Surf and Ice Beam are your reliable STAB attacks that hit hard. Always good. Hidden Power [Electric] hurts bulky Waters, whereas Hidden Power [Fire] can beat every Water Pokémon's worst nightmare, Ferrothorn. Explosion is less effective than last generation as it no longer halves Defence, but it can catch a special sponge off guard and really hammer them.

Being initially strong is always cool, especially with hugely powerful attacks like Icicle Spear and Rock Blast, which I'm sure you know how they work by now. Razor Shell is cool, but Hydro Pump can hit a wall like Skarmory on the switch. Explosion is powerful. Not quite as powerful as it was last generation, but it's still strong. However, Ice Shard hits faster Dragons hard.

Because of how hard Cloyster hits and the fact Icicle Spear and Rock Blast break through Substitute and Focus Sash, Cloyster can work as a lead. Ice Shard is the priority move to take them out in case they survived the first attack. Spikes sets up your own entry hazard, whereas Explosion is always cool as a last move.

EVs & Natures

Cloyster breaks the game by breaking itself
You could dump the 48 SAtk EVs into Speed or HP, as both are useful. However, 208 is the minimum Speed you should run as it beats ScarfChomp. 224 beats +1 base 105s. 244 beats +1 base 108s, and max Speed beats Gyarados, Dragonite and Blaziken before and after Shell Smash.

So outdated it's ridiculous
Max HP because Cloyster's base HP is pathetic. With a +Def nature, it doesn't need much investment at all. I just like 16 because it's a really early jump point. 8 Speed outspeeds Skarmory, meaning you can hit it with Surf or what not. The rest in Special Defence to give you some more bulk on the special side of things, helping you to take random Surfs.

Special Shell Smash
Same Speed numbers apply, so any extra you have put it into Attack for Explosion, or HP for bulk.

Offensive without Shell Smash
You don't really want to be taking hits, so running 4/252/252 is the most logical spread.

Lead
This set is designed to hurt, so that's what the EV spread is focused on.

Other Options

Normal move, Dive, Icicle Crash, Protect, Toxic
You could always run a Normal move to make the awesome Water/Normal attacking combo. However, it's pretty much a wasted move slot.Dive makes you immune to damage (except from Surf) for a turn, then attacks. It's an option to stall for Toxic damage, I guess.Icicle Crash is cool and all but... ICICLE SPEAR.Protect also stalls for Toxic damage.Toxic can be used on some Toxic stall set with Dive and Protect, or just as a move on the utility set.

Double & Triple Battle Options

Due to its only useful move being Shell Smash, which is really the only reason it is being used... in doubles and triples, Cloyster isn't too useful. Rock Slide, Earthquake, even Surf will be really hurting after Shell Smash, and as they target the entire screen, you can't do much to avoid them. Aside from Shell Smash, Cloyster isn't too useful in general. Sure, it has strong attacks and Explosion but... it has such lame HP and Special Defence, it'll be finished before it can do anything. Not to mention that defensively dire Ice type... stick to singles with Cloyster.

Partners

You want stuff that can either benefit from everything being decimated by Icicle Spear, or stuff that benefits Cloyster by removing any bulky Waters. Also, make sure you have something to keep the field clear of Stealth Rock and Spikes. They really ruin Cloyster's day. Forretress, Espeon, just keep those rocks away. Beating the bulky Waters, why not run Ferrothorn? It practically ruins every Water Pokémon in the game. Something to switch into Fighting Pokémon would be nice too, so maybe a Ghost? Zapdos helps with bulky Waters too. As Cloyster is superb at hurting... everything, you could run powerful Pokémon like Garchomp to clean up after you decimate with Icicle Spear. If Cloyster is your team's focus, keep entry hazards away, and banish bulky Waters back to the sea.

Countering Cloyster

Normally I'd say the standard walls like Skarmory and Ferrothorn, but they actually get OHKOed by Cloyster after Shell Smash. So... back to the drawing board. Straight away you will see bulky Waters are great for beating Cloyster. Rock Blast will still hurt unless you are Swampert or Quagsire, but you should be able to deal enough damage to beat it before you crumble. Having said that, if the Cloyster is running King's Rock, be prepared to get flinched. Probably the best counter for Cloyster is Empoleon. It resists everything Cloyster has and can hit it back hard. Sadly, Empoleon isn't as useful nowadays, but it is always there as an option. Cloyster's most common counter is probably Stealth Rock. It pretty much decimated Cloyster into the UU tier alone back in 4th gen. Priority also kills Cloyster. Yeah sure, you have boosted its offences and its Speed but... -2 in each of the defences? Hello Mach Punch and Vacuum Wave! Breloom, Conkeldurr, Infernape, Lucario, all of these Pokémon have a priority move that decimates Cloyster after a Shell Smash. Scizoralso does well, as Bullet Punch is crazy strong. Anything faster than Cloyster after it uses Shell Smash is in for a good chance of really, really hurting it. Scarf Starmie, +1 base 110s like Gengar, even hard physical hitters do well. Despite these weaknesses and its shortcomings, Cloyster is a huge, huge threat to the OU tier. It can OHKO a lot of the game after Shell Smash, and if you aren't prepared, it will beat you down and make you kiss its... shell. By no means perfect as it still has its counters and flaws, Cloyster is still an incredibly scary Pokémon that you need to think about when making a team.